"Class" specs aren't terribly equal, some manufacturers aren't exactly, shall we say, "honest" with their spec claims. In theory the class number is supposed to be the MB/Sec that is the _minimum_ sustained transfer rate the card is capable of, this is usually measured as read-speed which is always faster than write-speed.

At least, that's my understanding, feel free to correct me.

The USB interface of most mobile devices isn't fast enough to benefit much from the higher-end cards anyway. It pays to check reviews with benchmarks if you're worried about such things.

stevenz: "Class" specs aren't terribly equal, some manufacturers aren't exactly, shall we say, "honest" with their spec claims. In theory the class number is supposed to be the MB/Sec that is the _minimum_ sustained transfer rate the card is capable of, this is usually measured as read-speed which is always faster than write-speed.

At least, that's my understanding, feel free to correct me.

The USB interface of most mobile devices isn't fast enough to benefit much from the higher-end cards anyway. It pays to check reviews with benchmarks if you're worried about such things.

Turns out it's "measured" as write-speed, not read-speed as it turns out, oops.

stevenz: "Class" specs aren't terribly equal, some manufacturers aren't exactly, shall we say, "honest" with their spec claims. In theory the class number is supposed to be the MB/Sec that is the _minimum_ sustained transfer rate the card is capable of, this is usually measured as read-speed which is always faster than write-speed.

Here's what Wikipedia has to say about SD card class ratings:

The Speed Class Rating is the official unit of speed measurement for SD Cards, defined by the SD Association. The Class number represents a multiple of 8 Mbits/s (1 MB/s), and meets the least sustained write speeds for a card in a fragmented state.These are the ratings of all currently available cards:

Even though the class ratings are defined by a governing body, like "×" speed ratings, class speed ratings are quoted by the manufacturers and not verified by any independent evaluation process. In applications that require sustained write throughput, such as video recording, the device may not perform satisfactorily if the SD card's class rating falls below a particular speed. For example, a camcorder that is designed to record to class 6 media may suffer dropouts or corrupted video on slower media. On slower class cards, digital cameras may experience a lag of several seconds between photo-taking, while the camera writes the picture to the card.

Note the bit I've made bold + italic in the last paragraph. The more expensive brands (Sandisk, Lexar, Kingston, etc) tend to produce cards that follow the rating system, and usually provide a lot better speeds than their class rating would suggest. The cheaper brands tend to cheat and report class ratings that are higher than they should be.

For example, a cheap SD card maker might label a card as Class 6, but it only gets 6MB/s sustained writes on an empty card. Or it might get an average 7MB/s write speed over the course of several seconds, but with instantaneous speeds as low as 3MB/s.

In my experience you're better off buying a slightly lower class card from a more expensive manufacturer. Not only is the speed class more likely to be accurate, you're also less likely to get a dud card.